1. Field of the Invention
The present invention relates to a capacitive measuring device or apparatus; in particular, a length-measuring apparatus, and incorporates a scale portion and a sensing unit which is movable relative to the former and which supports transmitting electrodes and at least one receiving electrode. Herein, a transmission circuit applies transmitting signals at a base frequency to the transmitting electrodes, and with the receiving electrode being connected to a reception circuit, to the output side of which there is connected an evaluating circuit for the receiving signals.
2. Discussion of the Prior Art
For instance, capacitive measuring apparatuses of the above-mentioned kind are employed in connection with slide-rule gauges or caliper scales. The scale portion is formed by capacitor plates which provide for the capacitive coupling between the transmitting electrodes and the receiving electrode or electrodes, which are similarly formed by condenser coatings or capacitor plates. Upon a movement of the scanning or sensing unit relative to the scale portion, there changes the capacitive coupling between the transmitting electrodes and the receiving electrodes. This aspect is evaluated for displaying the positions of the displacement.
A measuring apparatus of the foregoing type is described in European Patent Appln. EP 0 184 584 B1. In that instance, three transmitting electrodes are supplied with three AC-voltages which are each phase-shifted by respectively 120.degree., and are especially sinusoidal in nature. These voltages can be generated in a transmitting circuit from rectangular or square-wave signals. However, it is also possible to contemplate a supply of digital signals which contain only harmonics of higher orders which can be readily filtered out.
In the disclosure of EP 0 184 584 B1 there is utilized the phase shift between the transmitting signal and the receiving signal, which is encountered during the displacement of the scanning or sensing unit relative to the scale portion, in order to determine the local or positional displacement. Inasmuch as the electronic components of the measuring apparatus themselves can lead to phase shifts, there must be counted upon the occurrence of errors in measurement, which are difficult to control. Moreover, it is also expensive or complex to generate a plurality of sinusoidal signals possessing the same amplitude and with a precise phase shift. In the case of rectangular or square-wave signals, the harmonics of high-amplitude give rise to comparatively large interpolation errors.
In addition to the foregoing, the above-mentioned known circuit is susceptible to external interference signals. The reception signals must, as a result, have to be shielded against possible interference signals; for example, including signals emanating from an LCD-display. This considerably increases the demand on expenditures for components, and therewith the manufacturing costs.
German Patent No. 28 53 142 describes another capacitive measuring apparatus for determining the relative position of two members which are mutually displaceable relative each other. In that apparatus, sinusoidal voltages are generated at the transmitting end by an oscillator, and these are then applied to the transmitting electrodes. The phase shift of the measurement signal at a displacement of the scanning or sensing unit relative to the scale portion, is evaluated for purposes of measurement. The direction of the displacement is determined through a counter whose count condition increases or lowers in dependence upon the displacement. In this instance, there are also encountered the difficulties as set forth in connection with EP 0 184 584 B1.
German Published Patent Appln. No. 22 18 824 also describes a method of measuring the displacement of an electrode of a differential capacitor relative to other electrodes.